This invention relates to a relay drive circuit, and more particularly, to a circuit for driving a relay used in an AC circuit, that includes protection against contact welding of the relay.
It is known in the art that a phenomenon called "contact welding" occurs in a relay when the relay is closed and opened many times, thereby repeatedly turning a DC circuit on and off. This contact welding phenomena is believed to develop in the following process.
A DC circuit is known which comprises a power source, and a relay connected in series to the power source. The relay has fixed and movable contacts, both made of copper. The movable contact comes into or out of contact with the fixed contact, in accordance with whether or not a voltage is applied from the power source to the relay. An overcurrent momentarily flows through the relay as the movable contact touches the fixed contact, or moves away therefrom. This overcurrent is dissipated as heat, which melts a portion of the movable contact made of copper. Part of the molten copper changes into positively charged copper ions. These ions move to the fixed contact, which is negatively charged. As the relay is closed and opened repeatedly, more and more copper ions move from the movable contact to the fixed contact. Finally, copper is deposited on the fixed contact, and the movable contact becomes welded to the fixed one. This undesirable welding is known as "contact welding".
Contact welding has been thought to take place only in a relay used in a DC circuit wherein copper ions move in one direction. It has not been considered a problem with a relay used in an AC circuit, wherein copper ions move back and forth between the fixed and movable contacts. Nonetheless, in the case of an AC circuit with a capacitive load, an overcurrent is generated when the relay closes, and the contacts of the relay are partly melted by the heat resulting from this overcurrent. In the case of an AC circuit with an inductive load, an arc is generated when the relay opens, and the contacts of the relay are partly melted by this arc. Namely, in both types of AC circuits, copper ions move in one direction every time a pulse current flows through the relay in synchronism with the power source frequency. Hence, contact welding is also a problem with AC circuits.
The contact welding causes a short-circuiting of the relay contacts. In some cases, this short-circuiting results in significant damage, such as the disconnection of the AC circuit from the power source and the burning of the load of the AC circuit. Therefore, there has been much demand for a relay used in an AC circuit, as well a DC circuit, which is protected from contact welding.